What all is different about the BFR heat shield compared to the shuttle? I assume there is much newer tech since the 70s, is it easier to refurbish?
Anyway, the rocket equation means if you wanted to slow down before reentry you would effectively need to accelerate to twice the final speed 0 to X then X to 0. That's simply not going to happen.
It's not (not even close). Falcon 9 first stage isn't reentering from orbital velocity.
It's essentially impossible, and if it weren't it would be terribly wasteful. Orbital velocity is going to be primarily shed by atmosphere.
What confuses people is you need to bleed off a little energy to start decent, and another tiny amount to land. But, between those points 99% of the energy is bleed off via air.
But, as you say Falcon does use air breaking.
PS: Compare the amount lifted to LEO with the weight of an empty rocket. You find the weight of the rocket is significant which is why they use multi stage rockets even though this means lifting multiple sets of engines.
In contrast, the Falcon 9 first stages are at suborbial speeds and don't have a heat shield, so they need to do a substantial re-entry burns to bring down their speed before they hit the atmosphere to avoid damage.
It’s true they fire the engines briefly at the top, but doing so to slow slightly from Mach 10 is very different vs slowing from Mach 10 to zero or Mach 22.7 to Mach 10. And this still significantly reduces the cargo they can take to LEO.
PS: Remeber kenetic energy is velocity ^ 2 so your bleeding off 1/5 the energy. Further, adiabatic heating is from the compression of gas, lower speeds means lower compression.
https://en.m.wikipedia.org/wiki/Sub-orbital_spaceflight
like the Mercury-Redstone mission Freedom 7 that achieved a peak speed of ~5,100 mph, less than Mach 7.
https://en.m.wikipedia.org/wiki/Mercury-Redstone_3
Also, to be clear, the re-entry burns are definitely necessary to avoid damage to the Falcon 9 first stage.
https://www.quora.com/Why-dont-rockets-burn-up-in-the-atmosp...
(You don't explicitly say otherwise, but one could misread your comment as suggesting that the re-entry burns are optional or negligible.) My main point was that the re-entry burns are about reducing the velocity significantly (30%, or whatever), not just for steering the trajectory into the atmosphere like for de-orbit burns.
With the current design and mission profile yes they need to reduce speed via thrust. But, a large chunk of this is Falcon 9's aerodynamic profile. Even if heat's not an issue long cylinders are not stable but you could build a rocket with a different profile. Remember, it's just fine without a heat shield (other than the upper stage / engine area) when being used as a rocket and peak velocity is at separation.
So, I agree with this design that burn is necessary, my point is in terms of design space it's one of many options.
Further, to be clear it's not a meaningful option when you start talking about orbital velocity. The shuttle kind of gave heat shields a bad rap, but they can be really light and simple. The shuttle's problem was trying to build a reusable heat shield at the limit of what was possible vs a huge range of much simpler designs.
This is a useful point for me to keep in mind, thanks!
The Shuttle's heatshield had to deflect significantly more energy because of the different landing approach.
Unfortunately, part of the design requirements was to return a large amount of mass from orbit which means it needed to be designed to dump a rather obscene amount of energy into the upper atmosphere. This meant it could not do a rapid decent and because it would overheat, and thus needed to stay in the upper atmosphere for a long time. Which meant it's thermal tiles needed to operate for much longer time periods requiring extreme amounts of thermal protection.
PS: The wings also added even more weight issues.